Salt tolerance at single cell level in giant-celled Characeae

The impact of salt stress on the physiology and the transcriptome of the model streptophyte green alga Chara braunii

Chara braunii is a model for early land plant evolution and terrestrialization. Salt stress has a profound effect on water and ion transport activities, thereby interacting with many other processes, including inorganic carbon acquisition for photosynthesis. In this study, we analyzed the impact of salt stress (5 practical salt units, PSU) on the physiology and gene expression in C. braunii. Photosynthesis was only slightly affected 6 h after salt addition and returned to control levels after 48 h. Several organic compounds such as proline, glutamate, sucrose, and 2-aminobutyrate accumulated in salt-treated thalli and might contribute to osmotic potential acclimation, whereas the amount of K+ decreased. We quantified transcript levels for 17,387 genes, of which 95 were up-regulated and 44 down-regulated after salt addition. Genes encoding proteins of the functional groups ion/solute transport and cell wall synthesis/modulation were enriched among the up-regulated genes 24-48 h after salt stress, indicating their role in osmotic acclimation. However, a homolog to land plant ERD4 osmosensors was transiently upregulated after 6 h, and phylogenetic analyses suggested that these sensors evolved in Charophyceae. Down-regulated genes were mainly related to photosynthesis and carbon metabolism/fixation, consistent with the observed lowered growth after extended cultivation. The changed expression of genes encoding proteins for inorganic carbon acquisition might be related to the impact of salt on ionic relations and inorganic carbon uptake. The results indicate that C. braunii can tolerate enhanced salt concentrations in a defined acclimation process, including distinct gene expression changes to achieve new metabolic homeostasis.

Electrical Signals at the Plasma Membrane and Their Influence on Chlorophyll Fluorescence of Chara Chloroplasts in vivo

Action potentials of plant cells are engaged in the regulation of many cell processes, including photosynthesis and cytoplasmic streaming. Excitable cells of characean algae submerged in a medium with an elevated K+ content are capable of generating hyperpolarizing electrical responses. These active responses of plasma membrane originate upon the passage of inward electric current comparable in strength to natural currents circulating in illuminated Chara internodes. So far, it remained unknown whether the hyperpolarizing electrical signals in Chara affect the photosynthetic activity. Here, we showed that the negative shift of cell membrane potential, which drives K+ influx into the cytoplasm, is accompanied by a delayed decrease in the actual yield of chlorophyll fluorescence F' and the maximal fluorescence yield Fm' under low background light (12.5 µmol m-2 s-1). The transient changes in F' and Fm' were evident only under illumination, which suggests their close relation to the photosynthetic energy conversion in chloroplasts. Passing the inward current caused an increase in pH at the cell surface (pHo), which reflected high H+/OH- conductance of the plasmalemma and indicated a decrease in cytoplasmic pH due to the H+ entry into the cell. The shifts in pHo arising in response to the first hyperpolarizing pulse disappeared upon repeated stimulation, thus indicating the long-term inactivation of plasmalemmal H+/OH- conductance. Suppression of plasmalemmal H+ fluxes did not abolish the hyperpolarizing responses and the analyzed changes in chlorophyll fluorescence. These results suggest that K+ fluxes between the extracellular medium, cytoplasm, and stroma are involved in the functional changes of chloroplasts reflected by transients of F' and Fm'.

Plasma membrane-chloroplast interactions activated by the hyperpolarizing response in characean cells

Signaling pathways in plant cells often comprise electrical phenomena developing at the plasma membrane. The action potentials in excitable plants like characean algae have a marked influence on photosynthetic electron transport and CO2 assimilation. The internodal cells of Characeae can also generate active electrical signals of a different type. The so called hyperpolarizing response develops under the passage of electric current whose strength is comparable to physiological currents circulating between nonuniform cell regions. The plasma membrane hyperpolarization is involved in multiple physiological events in aquatic and terrestrial plants. The hyperpolarizing response may represent an unexplored tool for studying the plasma membrane-chloroplast interactions in vivo. This study shows that the hyperpolarizing response of Chara australis internodes whose plasmalemma was preliminary converted into the K+-conductive state induces transient changes in maximal (Fm') and actual (F') fluorescence yields of chloroplasts in vivo. These fluorescence transients were light dependent, suggesting their relation to photosynthetic electron and H+ transport. The cell hyperpolarization promoted H+ influx that was inactivated after a single electric stimulus. The results indicate that the plasma membrane hyperpolarization drives transmembrane ion fluxes and modifies the ionic composition of cytoplasm, which indirectly (via envelope transporters) affects the pH of chloroplast stroma and chlorophyll fluorescence. Remarkably, the functioning of envelope ion transporters can be revealed in short-term experiments in vivo, without growing plants on solutions with various mineral compositions.

How Plants Tolerate Salt Stress

Soil salinization inhibits plant growth and seriously restricts food security and agricultural development. Excessive salt can cause ionic stress, osmotic stress, and ultimately oxidative stress in plants. Plants exclude excess salt from their cells to help maintain ionic homeostasis and stimulate phytohormone signaling pathways, thereby balancing growth and stress tolerance to enhance their survival. Continuous innovations in scientific research techniques have allowed great strides in understanding how plants actively resist salt stress. Here, we briefly summarize recent achievements in elucidating ionic homeostasis, osmotic stress regulation, oxidative stress regulation, and plant hormonal responses under salt stress. Such achievements lay the foundation for a comprehensive understanding of plant salt-tolerance mechanisms.

The cell biology of charophytes: Exploring the past and models for the future

Charophytes (Streptophyta) represent a diverse assemblage of extant green algae that are the sister lineage to land plants. About 500-600+ million years ago, a charophyte progenitor successfully colonized land and subsequently gave rise to land plants. Charophytes have diverse but relatively simple body plans that make them highly attractive organisms for many areas of biological research. At the cellular level, many charophytes have been used for deciphering cytoskeletal networks and their dynamics, membrane trafficking, extracellular matrix secretion, and cell division mechanisms. Some charophytes live in challenging habitats and have become excellent models for elucidating the cellular and molecular effects of various abiotic stressors on plant cells. Recent sequencing of several charophyte genomes has also opened doors for the dissection of biosynthetic and signaling pathways. While we are only in an infancy stage of elucidating the cell biology of charophytes, the future application of novel analytical methodologies in charophyte studies that include a broader survey of inclusive taxa will enhance our understanding of plant evolution and cell dynamics.

The role of ion-transporting proteins in the evolution of salt tolerance in charophyte algae

Species within the genus Chara have variable salinity tolerance. Their close evolutionary relationship with embryophytes makes their study crucial to understanding the evolution of salt tolerance and key evolutionary processes shared among the phyla. We examined salt-tolerant Chara longifolia and salt-sensitive Chara australis for mechanisms of salt tolerance and their potential role in adaptation to salt. We hypothesize that there are shared mechanisms similar to those in embryophytes, which assist in conferring salt tolerance in Chara, including a cation transporter (HKT), a Na⁺ /H⁺ antiport (NHX), a H⁺ -ATPase (AHA), and a Na⁺ -ATPase (ENA). Illumina transcriptomes were created using cultures grown in freshwater and exposed to salt stress. The presence of these candidate genes, identified by comparing with genes known from embryophytes, has been confirmed in both species of Chara, with the exception of ENA, present only in salt-tolerant C. longifolia. These transcriptomes provide evidence for the contribution of these mechanisms to differences in salt tolerance in the two species and for the independent evolution of the Na⁺ -ATPase. We also examined genes that may have played a role in important evolutionary processes, suggested by previous work on the Chara braunii genome. Among the genes examined, cellulose synthase protein (GT43) and response regulator (RRB) were confirmed in both species. Genes absent from all three Chara species were members of the GRAS family, microtubule-binding protein (TANGLED1), and auxin synthesizers (YUCCA, TAA). Results from this study shed light on the evolutionary relationship between Chara and embryophytes through confirmation of shared salt tolerance mechanisms, as well as unique mechanisms that do not occur in angiosperms.

Salinity-induced Changes in Gene Expression in the Streptophyte Alga Chara: The Critical Role of a Rare Na+ -ATPase

The primarily freshwater genus Chara is comprised of many species that exhibit a wide range of salinity tolerance. The range of salt tolerance provides a good platform for investigating the role of transport mechanisms in response to salt stress, and the close evolutionary relationship between Charophytes and land plants can provide broader insights. We investigated the response to salt stress of previously identified transport mechanisms in two species of Chara, Chara longifolia (salt-tolerant), and Chara australis (salt-sensitive): a cation transporter (HKT), a Na⁺ /H⁺ antiport (NHX), H⁺ -ATPase (AHA), and a Na⁺ -ATPase (ENA). The presence of these candidate genes has been confirmed in both species of Chara, with the exception of the Na⁺ -ATPase, which is present only in salt-tolerant Chara longifolia. Time-course Illumina transcriptomes were created using RNA from multiple time points (0, 6, 12, 24 and 48 h) after freshwater cultures for each species were exposed to salt stress. These transcriptomes verified our hypotheses of these mechanisms conferring salt tolerance in the two species examined and also aided in identification of specific transcripts representing our genes of interest in both species. The expression of these transcripts was validated through use of qPCR, in a similar experimental set-up used for the RNAseq data described above. The RNAseq and qPCR data showed significant changes of expression mechanisms in C. longifolia (respectively), a down-regulation of HKT and a substantial up-regulation of ENA. Significant responses to salt stress in salt-sensitive C. australis show up-regulation of NHX and AHA.

Melatonin improves rice salinity stress tolerance by NADPH oxidase-dependent control of the plasma membrane K+ transporters and K+ homeostasis

This study aimed to reveal the mechanistic basis of the melatonin-mediated amelioration of salinity stress in plants. Electrophysiological experiments revealed that melatonin decreased salt-induced K⁺ efflux (a critical determinant of plant salt tolerance) in a dose- and time-dependent manner and reduced sensitivity of the plasma membrane K⁺ -permeable channels to hydroxyl radicals. These beneficial effects of melatonin were abolished by NADPH oxidase blocker DPI. Transcriptome analyses revealed that melatonin induced 585 (448 up- and 137 down-regulated) and 59 (54 up- and 5 down-regulated) differentially expressed genes (DEGs) in the root tip and mature zone, respectively. The most noticeable changes in the root tip were melatonin-induced increase in the expression of several DEGs encoding respiratory burst NADPH oxidases (OsRBOHA and OsRBOHF), calcineurin B-like/calcineurin B-like-interacting protein kinase (OsCBL/OsCIPK), and calcium-dependent protein kinase (OsCDPK) under salt stress. Melatonin also enhanced the expression of potassium transporter genes (OsAKT1, OsHAK1, and OsHAK5). Taken together, these results indicate that melatonin improves salt tolerance in rice by enabling K⁺ retention in roots, and that the latter process is conferred by melatonin scavenging of hydroxyl radicals and a concurrent OsRBOHF-dependent ROS signalling required to activate stress-responsive genes and increase the expression of K⁺ uptake transporters in the root tip.

Chara braunii genome: a new resource for plant electrophysiology

The large-celled green alga Chara provided early electrophysiological data, but this model organism lost popularity once the smaller cells of higher plants became accessible to electrophysiology and genetic manipulation. However, with the sequencing of the Chara braunii genome (Nishiyama et al. Cell 174: 448-464, 2018), the molecular identity of the underlaying ion transporters in Characeae can be found and placed in evolutionary context. As Characeae are close to ancestors of land plants, the wealth of electrophysiological data will provide insights into important aspects of plant physiology, such as salt tolerance and sensitivity, carbon concentrating mechanisms, pH banding and the action potential generation.

Allelopathic effects of Chara species (C. aspera, C. baltica, and C. canescens) on the bloom-forming picocyanobacterium Synechococcus sp

The role of macroalgal allelopathy in aquatic systems has received increasing attention as a potential means of controlling cyanobacterial blooms. However, the allelopathic activity of Chara sp. on coexisting and bloom-forming picocyanobacteria is still largely unknown. Therefore, the laboratory experiments were conducted to investigate the allelopathic activity of extracts of Chara aspera, C. baltica, and C. canescens on the growth, the fluorescence parameters: maximum and effective quantum yield of photosystem II (PSII) photochemistry (F_v/F_m and ΦPSII, respectively) and photosynthesis parameters such as the initial slope of photosynthesis-irradiance (P-E) curves (alpha) and photosynthetic capacity (P_m) of the picocyanobacterium Synechococcus sp. Batch cultures of picocyanobacterium were exposed to three concentrations of extracts originating from three charophyte cultures and the effect was followed at three sampling times. Dried specimens of C. aspera, C. baltica, and C. canescens were extracted in the water-based matrix and the initial Synechococcus sp. inoculum, derived from unialgal culture media, was used. We found both negative and positive allelopathic effects of all tested Chara extracts on Synechococcus sp. The strongest adverse impact of picocyanobacterium growth was caused by C. baltica. This study clearly demonstrated that the allelopathic effect depends on the Chara species identity. Our results also suggested that some allelopathic Chara sp. have the potential to mitigate harmful cyanobacterial blooms in systems dominated by Synechococcus sp.

Transitions between marine and freshwater environments provide new clues about the origins of multicellular plants and algae

Marine-freshwater and freshwater-marine transitions have been key events in the evolution of life, and most major groups of organisms have independently undergone such events at least once in their history. Here, we first compile an inventory of bidirectional freshwater and marine transitions in multicellular photosynthetic eukaryotes. While green and red algae have mastered multiple transitions in both directions, brown algae have colonized freshwater on a maximum of six known occasions, and angiosperms have made the transition to marine environments only two or three times. Next, we review the early evolutionary events leading to the colonization of current habitats. It is commonly assumed that the conquest of land proceeded in a sequence from marine to freshwater habitats. However, recent evidence suggests that early photosynthetic eukaryotes may have arisen in subaerial or freshwater environments and only later colonized marine environments as hypersaline oceans were diluted to the contemporary level. Although this hypothesis remains speculative, it is important to keep these alternative scenarios in mind when interpreting the current habitat distribution of plants and algae. Finally, we discuss the roles of structural and functional adaptations of the cell wall, reactive oxygen species scavengers, osmoregulation, and reproduction. These are central for acclimatization to freshwater or to marine environments. We observe that successful transitions appear to have occurred more frequently in morphologically simple forms and conclude that, in addition to physiological studies of euryhaline species, comparative studies of closely related species fully adapted to one or the other environment are necessary to better understand the adaptive processes.

Multi-Scale Characean Experimental System: From Electrophysiology of Membrane Transporters to Cell-to-Cell Connectivity, Cytoplasmic Streaming and Auxin Metabolism

The morphology of characean algae could be mistaken for a higher plant: stem-like axes with leaf-like branchlets anchored in the soil by root-like rhizoids. However, all of these structures are made up of giant multinucleate cells separated by multicellular nodal complexes. The excised internodal cells survive long enough for the nodes to give rise to new thallus. The size of the internodes and their thick cytoplasmic layer minimize impalement injury and allow specific micro-electrode placement. The cell structure can be manipulated by centrifugation, perfusion of cell contents or creation of cytoplasmic droplets, allowing access to both vacuolar and cytoplasmic compartments and both sides of the cell membranes. Thousands of electrical measurements on intact or altered cells and cytoplasmic droplets laid down basis to modern plant electrophysiology. Furthermore, the giant internodal cells and whole thalli facilitate research into many other plant properties. As nutrients have to be transported from rhizoids to growing parts of the thallus and hormonal signals need to pass from cell to cell, Characeae possess very fast cytoplasmic streaming. The mechanism was resolved in the characean model. Plasmodesmata between the internodal cells and nodal complexes facilitate transport of ions, nutrients and photosynthates across the nodes. The internal structure was found to be similar to those of higher plants. Recent experiments suggest a strong circadian influence on metabolic pathways producing indole-3-acetic acid (IAA) and serotonin/melatonin. The review will discuss the impact of the characean models arising from fragments of cells, single cells, cell-to-cell transport or whole thalli on understanding of plant evolution and physiology.

Auxin effects on ion transport in Chara corallina

The plant hormone auxin has been widely studied with regard to synthesis, transport, signaling and functions among the land plants while there is still a lack of knowledge about the possible role for auxin regulation mechanisms in algae with "plant-like" structures. Here we use the alga Chara corallina as a model to study aspects of auxin signaling. In this respect we measured auxin on membrane potential changes and different ion fluxes (K(+), H(+)) through the plasma membrane. Results showed that auxin, mainly IAA, could hyperpolarize the membrane potential of C. corallina internodal cells. Ion flux measurements showed that the auxin-induced membrane potential change may be based on the change of K(+) permeability and/or channel activity rather than through the activation of proton pumps as known in land plants.

Genomic-assisted haplotype analysis and the development of high-throughput SNP markers for salinity tolerance in soybean

Soil salinity is a limiting factor of crop yield. The soybean is sensitive to soil salinity, and a dominant gene, Glyma03g32900 is primarily responsible for salt-tolerance. The identification of high throughput and robust markers as well as the deployment of salt-tolerant cultivars are effective approaches to minimize yield loss under saline conditions. We utilized high quality (15x) whole-genome resequencing (WGRS) on 106 diverse soybean lines and identified three major structural variants and allelic variation in the promoter and genic regions of the GmCHX1 gene. The discovery of single nucleotide polymorphisms (SNPs) associated with structural variants facilitated the design of six KASPar assays. Additionally, haplotype analysis and pedigree tracking of 93 U.S. ancestral lines were performed using publically available WGRS datasets. Identified SNP markers were validated, and a strong correlation was observed between the genotype and salt treatment phenotype (leaf scorch, chlorophyll content and Na(+) accumulation) using a panel of 104 soybean lines and, an interspecific bi-parental population (F8) from PI483463 x Hutcheson. These markers precisely identified salt-tolerant/sensitive genotypes (>91%), and different structural-variants (>98%). These SNP assays, supported by accurate phenotyping, haplotype analyses and pedigree tracking information, will accelerate marker-assisted selection programs to enhance the development of salt-tolerant soybean cultivars.